A mechanical seal 4 is a static pressure type non-contact gas seal, including a rotary sealing ring 5 that is fixed to a rotating shaft, and a stationary sealing ring 7 that is engaged and held in an inner periphery of a seal case 6 made of a metal by means of O-rings 12 so as to move in an axial direction thereof. The O-rings 12 are held engaging with the outer periphery of the stationary sealing ring 7, and come in press contact with the inner periphery of the seal case 6 so as to give a secondary seal to a gap between the stationary sealing ring 7 and the seal case 6, while allowing the stationary sealing ring 7 to move ih ng a thickness in the range of 5 to 100 μm is formed on the inner periphery of the seal case 6 at least within a range in which the O-rings 12 relatively move in accordance with the movement of the stationary sealing ring 7 in the axial direction.